1. Field of the Invention
The present invention relates to technique for controlling a transfer voltage in an image forming apparatus, and in particular, to a technique for detecting a composite resistance between an organic photo conductive (OPC) drum and a transfer roller before a sheet of recording paper advances between the OPC drum and the transfer roller, so as to control a transfer voltage according to the detected composite resistance.
2. Description of the Related Art
In general, an image forming apparatus charges a photoconductive layer of an OPC drum made of a photo-semiconductor such as zinc oxide or selenium, exposes the photoconductive layer according to an image signal to form an electrostatic latent image, develops the electrostatic latent image with a toner, and then transfers the developed toner image to the recording paper. The image forming apparatus employs a contact-charging technique, which is widely used to minimize generation of ozone due to charging by bring a conductive roller or brush serving as a contact charging device into contact with the OPC drum to form a uniform surface charge on the OPC drum. The image forming apparatus supplies a proper transfer voltage to a transfer roller in order to transfer the toner developed on the OPC drum to the recording paper without degradation of the image.
U.S. Pat. No. 5,682,575 to Komori, entitled ELECTROPHOTOGRAPHIC RECORDING APPARATUS HAVING TRANSFER VOLTAGE CONTROL DEVICE, discloses a technique for controlling the transfer voltage by detecting a resistance of the transfer roller when a leading end of the recording paper passes the transfer roller. In this transfer voltage control technique, a transfer voltage is determined when the leading end (.apprxeq.5 mm) of the recording paper, which is a non-image formative area, passes between the OPC drum and the transfer roller, in accordance with a composite resistance of the recording paper, the OPC drum and the transfer roller. However, this technique reads the composite resistance only at the leading end, i.e., the non-image formative area of the recording paper, so that this technique may not be suitable for a high speed image forming apparatus. That is, in a high speed laser printer, the non-image formative area advances too fast to read an accurate composite resistance, decreasing the transfer efficiency. Moreover, a voltage used for reading the composite resistance may be supplied even to an image formative area undesirably, resulting in the image degradation due to the decreased transfer efficiency.
U.S. Pat. No. 5,799,226 to Shigeta et al., entitled ELECTROSTATIC IMAGE FORMING APPARATUS WITH TRANSFER CONTROLS FOR DIFFERENT IMAGING MODES, discloses an imaging forming apparatus having various transfer and attraction voltages in which the transfer voltage is varied in accordance with the type of transfer paper.
The patent to Takemoto et al., U.S. Pat. No. 5,774,762, entitled IMAGE FORMING APPARATUS FOR OPTIMIZING TONER TRANSFER EFFICIENCY, discloses an image forming apparatus for optimizing the toner transfer efficiency in which the resistance of the transfer material is detected and the transfer voltage controlled in accordance with the resistance of the transfer material.
The patent to Takeda, U.S. Pat. No. 5,287,144, entitled IMAGE FORMING APPARATUS HAVING TRANSFER CHARGER WHICH IS CONTROLLED ACCORDING TO AMBIENT CONDITIONS, discloses an image forming apparatus having a transfer charger which is controlled according to ambient conditions. The transfer current is varied in accordance with the resistance of the transfer material when the transfer material passes between the contact member and a transfer material carrying sheet.
The following patent each discloses features in common with the present invention but are not as pertinent as the patents discussed above: U.S. Pat. No. 5,799,225 to Abe et al., entitled IMAGE FORMING APPARATUS HAVING VARIABLE TRANSFER AND ATTRACTION VOLTAGE, U.S. Pat. No. 5,822,651 to Yim et al., entitled TRANSFER VOLTAGE ADJUSTING DEVICE, U.S. Pat. No. 5,151,736 to Ohzeki et al., entitled IMAGE FORMING APPARATUS WITH CONTROLLED TRANSFER VOLTAGE, and U.S. Pat. No. 5,848,321 to Roh et al., entitled METHOD FOR AUTOMATICALLY CONTROLLING TRANSFER VOLTAGE IN PRINTER USING ELECTROPHOTOGRAPHY SYSTEM.